Promoting water dissociation for efficient solar driven CO2 electroreduction via improving hydroxyl adsorption

Exploring efficient electrocatalysts with fundamental understanding of the reaction mechanism is imperative in CO2 electroreduction. However, the impact of sluggish water dissociation as proton source and the surface species in reaction are still unclear. Herein, we report a strategy of promoting protonation in CO2 electroreduction by implementing oxygen vacancy engineering on Bi2O2CO3 over which high Faradaic efficiency of formate (above 90%) and large partial current density (162 mA cm−2) are achieved. Systematic study reveals that the production rate of formate is mainly hampered by water dissociation, while the introduction of oxygen vacancy accelerates water dissociation kinetics by strengthening hydroxyl adsorption and reduces the energetic span of CO2 electroreduction. Moreover, CO3* involved in formate formation as the key surface species is clearly identified by electron spin resonance measurements and designed in situ Raman spectroscopy study combined with isotopic labelling. Coupled with photovoltaic device, the solar to formate energy conversion efficiency reaches as high as 13.3%.

1. The authors have reported different techniques such as kinetic isotope effect (KIE) over the different catalysts to show that water dissociation kinetics is altered with VO engineered samples. However, cations have been shown to play a major role in water dissociation kinetics (pubs.acs.org/doi/10.1021/jacs.1c10171) with weakly hydrated cations showing higher activity. To further prove their hypothesis, the authors should report the performance of HER for BOC-C and BOC-2 samples in a different electrolyte such as NaHCO3 and discuss the interactions between cations and water molecule. 4. Fig. 6 (Supplementary) and Table 1 (Supplementary) : The authors show and quantify oxygen vacancy by XPS analysis. However, there have been reports on the wrong use of attributing O1s signal at 531-532 eV to oxygen vacancies (https://doi.org/10.1016/j.susc.2021.121894). The article states that ex-situ XPS measurements of oxygen are not possible. The authors should clarify this and provide additional characterization technique (EPS or XAFS) to show the oxygen vacancies. 5. In Fig.1k, there are no significant differences in the intensity of Bi 4f for BOC-C and the four BOC with V0 engineered samples. What is the reason for this? Shouldn't the VO engineered samples have a relatively higher percentage of Bi+/Bi than Bi3+ compared to that of BOC-C? The authors should explain this. 6. From EXAFS study reported, the authors should calculate and report the coordination number of Bi3+ and oxygen atoms for the 4 samples and compare it with bare BOC sample to further verify the differences in oxygen vacancies. 7. In Line 118, the authors state: "the selectivity and activity as function of potentials were testified by chronopotentiometry". Product selectivity from CO2RR are calculated through in-line gas chromotagraphy and NMR spectroscopy for liquid products. Here, details on how the faradaic efficiency (FE) of formate, CO and H2 were quantified are missing. Sample calculation of formate from NMR and CO and H2 from GC analysis must be reported. 8. Fig.S17: The authors show proton NMR spectra for formate. If possible, the authors must include 13C NMR results as well. 9. BET surface area measurements for all the samples must be provided to identify the differences in surface area and porosity of the samples.
10. Line 143-144: Details on the experimental setup for flow-cell system using gas diffusion electrode (GDE) are not reported. The authors should provide details on how the catalysts were prepared and deposited on a carbon GDL. In addition, details on CO2 flowrate and stability of GDE must be reported at 200 mA/cm2. 11. Additionally, double layer capacitance measurements must be provided for flow cell tests to report ECSA and turn over frequency of catalysts. This data must be compared with previous literature reports on other GDE catalysts (Sn, In) for formate production at high current densities (although the authors have compared the values in literature for H-cells in Table S3 and S4). Discussion on why BOC-2 might be beneficial at these high current densities should be added.
12. For the GDE tests in flow cells, wouldn't the BOC-VO samples reduce to metallic Bi at these high current densities? If that's the case, how does the performance of BOC-2 compare with bare BOC-C in these flow cells at high current densities? The authors should report the performance of bare BOC-C sample as well in flow cells at high current densities to discuss if there are any significant differences or if the observed differences in formate production for BOC-C and BOC-2 occurs only in H-cells. This is because at current densities > 50mA/cm2, the local pH >12 which might alter the water dissociation kinetics. Discussion on this aspect must be included.
13. Since the authors study the correlation between oxygen vacancy (VO) and formate production, providing additional numbers on optimal Vo concentration for a given electrode size and current density would be a useful metric.
exchange. The authors should discuss this and what this means in the context of their hypothesis.

Reviewer #3 (Remarks to the Author):
The authors investigate the effect of oxygen vacancies in Bi2O2CO3 (BOC) on enhancing the CO2 reduction reaction to formate by boosting the water dissociation. The experimental results and theoretical calculation demonstrate that oxygen vacancies stabilize the hydroxyl group and drive efficient water dissociation. Consequently, the energy barrier for *OCHO intermediate is reduced on oxygen sites of BOC. This work is done well and clearly shows the evidence for their claim. However, the reviewer can find one major inconsistency in BOC electrocatalysts compared to the previous publications and minor issues that should be thoroughly addressed via major revision. 1) In Figure S1, there are several XRD peaks. BOC nanosheets appear to be polycrystalline rather than single crystalline.
2) The reviewer cannot find the defect in Figures 1d and S5. Please explain how to identify one.
3) In Figure 1g, the y-axis values show less than 3% difference in intensity that might be within the experimental error. Also, why the presence of oxygen vacancy shows a larger intensity in the y-axis than that of oxygen sites? 4) Why does controlling the water:ethylene glycol ratio change the oxygen vacancies concentration in BOC? A brief description should be at least provided. 5) If the water dissociation is promoted, hydrogen evolution reaction (HER) might also be enhanced due to the increase in the number of protons near the surface. So, please discuss why the competing HER is not enhanced but only CO2 reduction is enhanced. 6) Authors claim that "there is no obvious change in phase structure, morphology and valance state, which demonstrates the stability of BOC-2 ( Supplementary Fig. 15)." However, this is in great contradiction with the recently reported BOC catalyst (https://doi.org/10.1002/eem2.12490). Based on the Pourbaix diagram of BOC, it should be reduced to Bi under cathodic bias (https://doi.org/10.1016/S0378-7753(00)00640-6). Authors should clearly state the reason for the discrepancy by citing the above references.
7) The reviewer also wants to know and recommend explaining why H2O showed a higher formate production rate compared to D2O  Response: We thank the reviewer for this comment. The HER performance of BOC-C and BOC-2 were evaluated specifically in Ar-saturated NaHCO3 solutions as suggested. As can be seen from Figure R1, the overpotential of BOC is lower in KHCO3 than that in NaHCO3, which is consistent with the result reported by Koper et al. and indicates that cation is indeed effective in water dissociation (J. Am. Chem. Soc. 144, 1589-1602 (2022); this work has been cited in our revised manuscript as Ref. 61). However, less enhancement induced by cations is observed on BOC-2, which can be attributed to the intrinsic faster water dissociation kinetics due to VO introduction.
Meanwhile, in addition to KIE, the remarkable enhancement of water dissociation in CO2RR promoted by VO is also validated at different local pH while the cation concentration is the same. As shown in Figure R2, the formate production rate of BOC-2 with optimized VO concentration is much higher.
These results demonstrate that the introduction of VO is able to accelerate water dissociation kinetics which is in favor of formate production as a result.

Changes have been made in the revised manuscript (Page 9, Line 192):
"… These results agree well with the formate production rates dissused above and a strong correlation among VO, water dissociation ability and formate production is identified. Besides, less enhancement in activity induced by cations can be observed on BOC-2 as compared with that on BOC-C, indicating the intrinsic faster water dissociation kinetics of BOC-2 ( Supplementary Fig.30)

Response:
We thank the reviewer for this important comment and apologize for the imprecise expression in original manuscript. To understand the electrochemical interface and the role of VO thoroughly, the EIS measurement of BOC-C was implemented complementarily with the frequency range of 1-1000000 HZ and amplitude of 5 mV. All the EIS data were fitted with equivalent circuit consisting of two sections based on previous studies (Nano Energy 71, 104653 (2020); Angew. Chem. Int. Ed. 61, e202206077 (2022)), over which the fitted results match well with experimental data. As shown in Figure R3, the first section represents the uncompensated solution resistance (Rs) while the second section reflects the interfacial properties including charge transfer resistance (Rct) and the resistance of adsorbed intermediates including water and hydroxyl (Rp). The equivalent circuit parameters are calculated and summarized in Table R1.
With the introduction of VO, the charge transfer of BOC-2 is accelerated with smaller Rct value as compared with that of BOC-C, which can be in favor of reducing the overpotential and increasing the conductivity in CO2RR. This deduction is supported by the results of performance evaluation during CO2RR ( Figure R4). However, the introduction of excessive VO (BOC-4) limits the charge transfer rate, which may be resulted from the structure distortion.
In addition, it can be noted that the resistance induced by water and hydroxyl adsorption (Rp) is related with VO concentration, which implies that VO is able to change water and hydroxyl adsorption.
According to our KIE experiments and DFT analysis ( Figure R5), the introduction of VO can strengthen hydroxyl adsorption and expediate water dissociation kinetics, which contributes to reducing the energetic span in formate formation. Overall, the role of VO in BOC can be concluded to promote water dissociation accompanied by accelerating charge transfer, which is beneficial for reducing the overpotential and boosting formation production rate in CO2RR.

Changes have been made in the revised manuscript (Page 7, Line 170):
" … Next, electrochemical impendence spectroscopy (EIS) was carried out and fitted by equivalent circuit to investigate the electrochemical interface properties 58-59 . With the introduction of VO, the charge transfer can be accelerated, which is beneficial for conductivity and reducing the overpotential (Supplementary Fig. 25 and Supplementary Table 6). Besides, the resistance induced by water and hydroxyl adsorption (Rp) is related with VO concentration, implying that VO might affect the proton transfer by changing water and hydroxyl adsorption. Before conducting the investigation on proton transfer further, the proton source was clarified through isotopic labelling of D2O. …"   and black line represent samples before and after annealing, respectively. b The free energy of step Ⅲ in formate formation, ΔμOH−w, the adsorption energy of OH* and H2O*.  Figure R6. Then, the ECSA of different samples were calculated based on the following equation: Among this, S stands for the real surface area of smooth electrode, equivalent to the geometric area of work electrode (1 cm 2 in our case). R is the roughness factor, which is equal to the ratio of C and general specific capacitance of metal oxide (C , 60 μF cm −2 , Angew. Chem. Int. Ed. 59, 10807-1081 (2020); ACS Catal. 10, 743-750 (2020)). The corresponding values of C and ECSA for different samples are summarized in Table R2. It can be found that the introduction of VO slightly increases the ECSA, which should be helpful for activity.
To clarify the effect of ECSA and get more insight into the intrinsic activity of BOC with VO, the partial current of formate of different samples were normalized by ECSA. As can be seen from Figure   R7, the jformate ECSA normalized of BOC-2 is much larger than that of BOC-C, indicating the introduction of VO is notably in favor of elevating the intrinsic activity of BOC apart from the increased ECSA.

Changes have been made in the revised manuscript (Page 7, Line 161):
" … To clarify the intrinsic activity of BOC with VO, the jformate of different samples were normalized by specific surface area (Supplementary Fig. 21 and 22) and electrochemical surface area, respectively (Supplementary Fig. 23 and Supplementary Table 5). It can be found that the introduction of VO notably increases the intrinsic activity and the degree of this enhancement is closely related with the VO contents ( Supplementary Fig. 24). …" . Figure R7. The partial current density of formate normalized by ECSA for different catalysts (corresponding to Supplementary Fig. 24a in the revised Supplementary Information). In order to clarify the presence of VO further, electron paramagnetic resonance spectroscopy (EPR) measurement was carried out as it is considered as an effective method to detect VO based on its sensitivity on unpaired electrons. As shown in Figure R8, the signal with the g=2.002 which is the characteristic g value of VO can be observed, demonstrating the presence of VO strongly. Compared with BOC-C, the intensity of this typical signal increases in the sequence of BOC-1, BOC-2, BOC-3 and BOC-4, which suggests that the BOC with a series of VO concentration were obtained. The VO concentration was subsequently quantified through external standard method while 2,2,6,6tetramethylpiperidinyloxyl (TEMPO) was used as standard (Appl. Catal. B Environ. 224, 612-620 (2018); Sol. RRL 4, 2000037 (2020)) and the atomic VO concentration were estimated to be 0.031%, 0.044%, 0.060%, 0.076% for BOC-1, BOC-2, BOC-3 and BOC-4, respectively ( Figure R9 and Table   R3).
With the purpose of getting in-depth information about the VO location and coordinated structure, X-ray absorption spectroscopy including EXAFS and XANES was carried out. The coordination number of Bi-O for BOC with VO manifests a decrease with the rise of VO content, which suggests the presence of unsaturated Bi atoms induced by VO and coincides with our STEM results ( Figure R10 and Changes have been made in the revised manuscript (Page 5, Line 98): " … The regular variation of the sharp signal intensity with g value of 2.002 in EPR characterization not only further clarifies the presence of VO but also indicates that VO concentration increases in the sequence of BOC-1, BOC-2, BOC-3 and BOC-4 ( Fig. 1h) 49 . Commercial BOC (denoted as BOC-C) was adopted for comparison, and the VO concentration is much lower than that of BOC-1 as suggested by the weak EPR signal. The VO concentration was also quantified from EPR while atomic VO contents are estimated to be 0.031%, 0.044%, 0.060% and 0.076% for BOC-1, BOC-2, BOC-3 and BOC-4, respectively (Supplementary Table 1) 50 . The location of VO is disclosed from extended X-ray absorption fine structure (EXAFS) spectroscopy, and two peaks at around 1.6 Å and 3.       (2022)). Generally, the contents of the certain element can be semi-quantified by relative intensity in XPS spectra rather than the absolute intensity which is influenced by many factors such as surface finish of the sample, the status of XPS equipment, the chemical environment of elements and so on.
With the kind mind of this reviewer, the peaks in Bi 4f are re-fitted, and the peaks of 164.6 and 159.3 are assigned to Bi 3+ while the peaks of 164.0 and 158.5 correspond to Bi (3−x)+ (Ceram. Int. 48, 22163-22171 (2022)). As can be seen from Figure R12, the intensity of Bi (3−x)+ rises with the increase of VO concentration, indicating a relatively higher ratio of lower valance state of Bi due to the introduction of VO. Besides, the XANES absorption edge of BOC with VO shifts towards lower binding energy, which implies that BOC with VO has relative lower valance state on average and matches with the XPS results discussed above ( Figure R13). Figure R12. XPS spectra of Bi 4f for BOC-C, BOC-1, BOC-2, BOC-3 and BOC-4, respectively. The red area represents the peak of Bi (3−x)+ while the green area is the peak of Bi 3+ (corresponding to Fig. 1k in the revised manuscript).

From EXAFS study reported, the authors should calculate and report the coordination number of
Bi 3+ and oxygen atoms for the 4 samples and compare it with bare BOC sample to further verify the differences in oxygen vacancies.

Response:
We thank the reviewer for this comment. We are really sorry that the XAS data of BOC-4 and BOC-C are in absence in revision due to the limitation of operation period of XAS which is affected by the equipment maintenance schedule and Covid-19 prevention. Fortunately, the BOC-1 was complementarily characterized and the corresponding coordination numbers of Bi-O for BOC-1, BOC-2 and BOC-3 were calculated and summarized in Figure R14a and Table R5.    Response: Thank you for this kind reminder. The gas products from CO2RR were analyzed by a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). The Faradaic efficiency for gas products (including CO and H2) were calculated by the following equation: where Q is the number of electric charge for CO and H2, and Q represents the total number of electric charge during the whole CO2RR process. z represents the number of transfer electrons for producing a molecular product, which is 2 and 2 for CO and H2, respectively. v is the flow rate of gas products through GC and c stands for the concentration of gaseous products. F is the Faraday constant (96485 C mol −1 ) and V is the gas molar volume (24 L mol −1 at the condition of T=20℃, is the recorded total current. As for liquid product (formate), 1 H nuclear magnetic resonance (NMR) spectra (Avance III 400 MHz, Bruker, Germany) was used to quantify the production of formate. The Faradaic efficiency of formate was calculated by the following equation: where Q is the number of electric charge for formate and Q is the total number of electric charge in CO2RR. zformate is the number of transfer electrons for producing a molecular formate, which is 2. N represents the moles of formate based on the stand curve line of 1 H NMR.
is the total current on average during a fixed time (t) recorded by electrochemical working station and F is the Faraday constant (96485 C mol −1 ).

Changes have been made in the revised manuscript (Page 16, Line 369):
"… With regard to reflecting the CO2RR activity of different catalysts, chronoamperometry was performed at a series of potentials for at least an hour. The gas-phase products were analyzed online every 20 minutes by a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). The Faradaic efficiency for gas products (including CO and H2) were calculated by the following equation: where Q is the number of electric charge for CO and H2, and Q represents the total number of electric charge during the whole CO2RR process. z represents the number of transfer electrons for producing a molecular product, which is 2 and 2 for CO and H2, respectively. v is the flow rate of gas products through GC and c stands for the concentration of gaseous products. F is the Faraday constant (96485 C mol −1 ) and V is the gas molar volume (24 L mol −1 at the condition of T=20℃,

P=101.3 kPa).
is the recorded total current.
As for liquid product (formate), 1 H nuclear magnetic resonance (NMR) spectra (Avance III 400 MHz, Bruker, Germany) was used to quantify the production of formate. In detail, 400 μL of electrolyte after reaction was collected and mixed with 100 μL of D2O and DMSO after dilution (100 ppm) as the internal stand. The concentration of formate was calculated based on the calibration curve from a series of concentrations of standard HCOOK solutions. The Faradaic efficiency of formate was calculated by the following equation: where Q is the number of electric charge for formate and Q is the total number of electric charge in CO2RR. zformate is the number of transfer electrons for producing a molecular formate, which is 2. N represents the moles of formate based on the stand curve line of 1 H NMR.
is the total current on average during a fixed time (t) recorded by electrochemical working station and F is the Faraday constant (96485 C mol −1 ). …" 8. Fig.S17: The authors show proton NMR spectra for formate. If possible, the authors must include 13 C NMR results as well.

Response:
We are thankful to this reviewer's comment and the 13 C NMR spectra were obtained with 13 CO2 as the feed gas during CO2RR ( Figure R15). The signal at 172 ppm is attributed to H 13 COO − , demonstrating that the formate is indeed originate from CO2.

Changes have been made in revised manuscript (Page 7, Line 154):
"… Moreover, 13 CO2 labeling experiment was carried out. The proton doublet resulting from H-13 C coupling and H 13 COO − is observed in 1 H NMR and 13 C NMR, respectively (Supplementary Fig.   19), demonstrating that the produced formate derives from CO2 53 . …" Figure R15. The 13 C NMR spectra with 13 CO2 as carbon source during CO2RR (corresponding to Supplementary Fig. 19c in the revised Supplementary Information).
. 9. BET surface area measurements for all the samples must be provided to identify the differences in surface area and porosity of the samples.
Response: Thanks for this valuable comment. Considering the porous structure of BOC, N2 adsorption/desorption isotherms were supplemented to study the specific surface area and pore size distribution of different samples. As shown in Figure  To clarify the effect of BET surface in the enhancement of formate production, the jformate curves were normalized by the BET surface area, where the intrinsic activity of BOC-2 is notably higher compared with that of other samples ( Figure R18). This result demonstrates the specific surface area has negligible effect on the difference of formate production rate in our work.

Changes have been made in the revised manuscript (Page 7, Line 161):
"… To clarify the intrinsic activity of BOC with VO, the jformate of different samples were normalized  Response: We appreciate the reviewer's helpful comment. The GDE was prepared by electrodepositing BOC onto commercial carbon paper with hydrophobic microporous layer (Sigracet, Fuel cell store).
Specifically, the bare GDL of this carbon paper was covered by Kapton tape to avoid the blockage of microporous channel. Then, the BOC-GDE was prepared in the same process as described in methods section. The flow cell experiments were carried out in a commercial cell (GaossUnion, Tianjin, China).
The synthesized BOC-GDE was used as cathode while Ni foam was selected as anode.   Table S3 and S4). Discussion on why BOC-2 might be beneficial at these high current densities should be added.

Response:
We thank the reviewer for this comment. The Cdl was measured in flow cell with 1 M KOH as electrolyte ( Figure R20). As summarized in Table R6, the ECSA of BOC-2 is larger than that of where is the total current density; FE is the corresponding Faradaic efficiency; z is the number of transfer electrons, which is 2; F is the Faraday constant (96485 C mol −1 ); n is the mole of active sites and all Bi atoms were assumed to be active sites here.
In light of the reviewer's suggestion, the performance of our BOC-2 in flow-cell system was further compared with other reported electrocatalysts in formate production including Bi-based, Snbased and In-based materials as summarized in Table R7. It's worth noting that there are a few works with reported TOF value in flow cell as far as we known. Therefore, other important works with outstanding performance in flow-cell are also listed even though the TOF values are in absence. As can be seen from Table R7, the BOC-2 exhibits good performance at high current density, which can be attributed to the superior water dissociation on BOC-2 with VO introduction. Due to the higher current density at more negative potentials, the protons are consumed rapidly and the local pH of electrode can be alkaline which will make water dissociation more difficult. However, the introduction of VO has been demonstrated in our work to be notably in favor of water dissociation by reducing the energetic barrier through strengthening hydroxyl adsorption, which promotes formate production. Detailed investigation about the effect of VO at high current density is discussed in comment 12.

Changes have been made in the revised manuscript (Page 7, Line 146):
"… To relieve the limitation on mass transfer and pursue commercial current density requirements, BOC-2 was further integrated into gas diffusion electrode (GDE) and evaluated in flow-cell system 53 .
The jformate of BOC-2 is 1.6 times as high as that of BOC-C at −1.68 V while the turnover frequency can be up to 0.72 s −1 at 200 mA cm −2 , which demonstrates the activity enhancement through introducing VO can also be achieved in flow-cell system (Supplementary Fig. 17 and Supplementary Table 4). …" (Page 9, Line 200): "… As can be seen from Fig. 3d and Supplementary Fig. 31, the production rate of formate for BOC-2 is much higher than that of other samples at high local pH value, which indicates the beneficial effect of VO. Moreover, it can be found that BOC-2 shows superior activity and selectivity than BOC-C in flow-cell at large currenst density where water dissociation is more difficult due to the high local pH induced by rapid protons depletion ( Supplementary Fig.17 c, d). So, it can be concluded that water dissociation is involved in the rate-determining step for CO2RR to formate, and the presence of VO remarkably boosts the performance of CO2RR by accelerating H2O dissociation. …" Figure R20. CV curves and the derived Cdl value of different catalysts as GDEs. The turnover frequency (TOF) of BOC-2 was calculated according to the equation as follows where is the total current density; FE is the corresponding Faradaic efficiency; z is the number of transfer electrons, which is 2; F is the Faraday constant (96485 C mol −1 ); n is the mole of active sites and all Bi atoms in electrode were assumed to be active sites in our work. The n is calculated based on the equation as follows: where, x is the metallic composition in electrocatalyst, m . is the mass of electrocatalyst and M is the relative atomic mass of Bi. The TOF cited in this  That is, the Bi 3+ tends to be reduced to metallic Bi in thermodynamics at negative potentials while Bi 0 is easily to be oxidized to BOC during CO2RR due to high local pH and accessible CO2 adsorption.
In light of the reviewer's suggestions, the performance of BOC-C and BOC-2 were studied additionally in flow cell to clarify the role of VO at high current density. As can be seen from Figure   R25, the BOC-2 exhibits larger partial current density of formate compared with BOC-C at more negative potentials and the jformate of BOC-2 is 1.6 times as high as that of BOC-C at −1.68 V. As far as we known, the kinetics of water dissociation will be sluggish due to quick consumption of protons at high current density accompanied by high local pH, which will limit the reaction rate of CO2RR. Even though, BOC-2 with VO still exhibits superior ability in formate production, which can be attributed to the improved water dissociation induced by VO. Similar results can be observed by applying a series of current density on electrode, where BOC-2 shows higher FEformate relative to BOC-C ( Figure R25d).
Thus, the enhancement in activity through introducing VO is also notable at high current density, beneficial from the accelerated water dissociation kinetics.
Besides, the effect of local pH was investigated in our original manuscript by using three kinds of solutions with different buffer capacity (K2HPO4, K2CO3 and K2SO4) as electrolytes, which is responsible for varying different local pH around electrode. The production rate of fomate for BOC-2 is much higher at high local pH environment ( Figure R26). Overall, BOC-2 with VO exhibits superior activity in formate production compared with BOC-C at high current density where water dissociation is more difficult, which confirms that the introduction of VO can promotes formate formation by accelerating water dissociation kinetics.

Changes have been made in the revised manuscript (Page 7, Line 150):
"… In addition, a stable FEformate of 80% at large current density of 200 mA cm −2 can be observed during 15 h, which indicates the potential for practical application (Supplementary Fig. 18 "… As can be seen from Fig. 3d and Supplementary Fig. 31, the production rate of formate for BOC-2 is much higher than that of other samples at high local pH value, which indicates the beneficial effect of VO. Moreover, it can be found that BOC-2 shows superior activity and selectivity than BOC-C in flow-cell at large currenst density where water dissociation is more difficult due to the high local pH induced by rapid protons depletion ( Supplementary Fig.17 c, d). So, it can be concluded that water dissociation is involved in the rate-determining step for CO2RR to formate, and the presence of VO remarkably boosts the performance of CO2RR by accelerating H2O dissociation. …"

Since the authors study the correlation between oxygen vacancy (VO) and formate production,
providing additional numbers on optimal Vo concentration for a given electrode size and current density would be a useful metric.
Changes have been made in the revised manuscript (Page 5, Line 98): "… The regular variation of the sharp signal intensity with g value of 2.002 in EPR characterization not only further clarifies the presence of VO but also indicates that VO concentration increases in the sequence of BOC-1, BOC-2, BOC-3 and BOC-4 ( Fig. 1h) 49 . Commercial BOC (denoted as BOC-C) was adopted for comparison, and the VO concentration is much lower than that of BOC-1 as suggested by the weak EPR signal. The VO concentration was also quantified from EPR while atomic VO contents are estimated to be 0.031%, 0.044%, 0.060% and 0.076% for BOC-1, BOC-2, BOC-3 and BOC-4, respectively (Supplementary Table 1) 50 . The location of VO is disclosed from extended X-ray absorption fine structure (EXAFS) spectroscopy, and two peaks at around 1.6 Å and 3.5 Å corresponding to the scattering path of Bi-O and Bi-O-C are found (Fig. 1i, Supplementary Fig. 7 and Supplementary Table   2) …". Figure R27. EPR spectrum of TEMPO which was used as the standard to quantify the concentration of VO in BOC samples. (2020)) which gave us lots of inspiration to implement our work and were included in our supplementary information in our previous version. To emphasize the importance of these studies, they are cited in our revised text.

Changes have been made in the revised manuscript (Page 2, Line 50):
"… For electrode materials, metal oxide attracts broad attentions due to high selectivity and low overpotentials which are the crucial parameters for commercial scale in CO2RR to formate 20-28 .

(Page 3, Line 62):
"…Previous studies propose that CO2 tends to adsorb on oxygen site in metal oxide to form CO3* and participates in sequent reduction process as the key surface species, which is considered to be the origin of attractive performance for metal oxide electrocatalysts 39-45 . …"

…"
Minor comments 1. In Fig.2(a), the colors used for the plotting LSV curves in Ar saturated and CO2 saturated electrolyte solution can be improved and using thicker lines will make it easier for the reader to clearly see the differences for the 5 samples.

Response:
We appreciate the reviewer's kind suggestion and the LSV curves in Figure 2a are now plotted by using thicker lines to distinguish the activity of different samples more clearly.  Response: We thank the reviewer for this comment and we have corrected this ignoration in our revised manuscript.

Changes have been made in the revised manuscript (Page 3, Line 74):
"… Theoretical analysis shows CO3* participates in the formation of formate as the key surface species, which is demonstrated clearly through electron spin resonance (ESR) measurements and in situ Raman spectroscopy combined with isotopic labelling. Finally, full-cell electrocatalysis coupled with solar cell was constructed and achieves the solar to formate energy conversion efficiency of 13.3%. …"

Reviewer 2
The authors report the metal oxide type material Bi2O2CO3 as CO2RR catalyst. The overall presentation of gathered experimental data and its discussion is well done. However, interpretation of individual data set seems to be overstretched. Clarification and more critical assessment of the data sets is needed to support the mechanistic claims.

Overall, the direct link of the role of water dissociation to CO2RR activity is of interest for the scientific community. If the manuscript could be revised to include a critical assessment of the experimental data sets that supports the mechanistic interpretation, it should be published in Nat Comm.
Response: We really appreciate the reviewer's great endeavors to review our work. These positive and constructive comments could help us get thorough insight into our work. In this revision, additional experiments and discussion are included based on the reviewer's suggestions and concerns.

Specific comments: Comment 1: EIS analysis
EIS spectroscopy has been carried out the investigate the effect of VO on the conductivity and hence resistivity of the material (SI Figure S16). In fact, the EIS data is significantly different between BOC-

to BOC-4. The interpretation that the conductivity of the materials did not change is not valid. Also, the EIS data indicates that the electrochemical interface of the material is different. This should have
an impact on electrochemical reactions proceeding at the interface of the respective material. A more thorough discussion of the data is therefore needed to account for these differences. First, the plot SI Figure 16 lacks the BOC-C, which is cardinal to allow for comparison with the unmodified material.
Furthermore, the x axis should also be expanded to show the behaviour at the higher resistances.
Second, the resistances of the dielectric layer are shifted to different values. Also, the circles do not fully close, particularly for the BOC-1 material, which behaves completely different to the other materials.

Response:
We thank the reviewer for his or her insightful suggestion and apologize for the imprecise expression in original manuscript. Under the reviewer's guidance, the EIS data of BOC-C was added and the x axis was expanded to analysis the EIS data more thoroughly as shown in Figure R29 and Table R9. The arc shape of BOC-1 can be attributed to the dispersion effect due to the roughness, inhomogeneity and porous structure of stacked nanosheets.
In order to investigate the electrochemical interface properties, the equivalent circuit was employed to fit the impendence result as shown in Figure R29 (2) the second section reflects interfacial resistance at electrode-electrolyte interface including Rct (charge transfer process) and Rp (water and hydroxyl adsorption). With the introduction of VO, the charge transfer resistance of BOC-2 is lower than that of BOC-C, which indicates that VO is able to optimize the conductivity of BOC and accelerate the charge transfer rate. This faster charge transfer is conductive to the lower overpotential of BOC with VO as shown in Figure R30. However, BOC with much more VO (BOC-4) shows larger Rct, which might be resulted from the structure distortion.
Apart from the variation in Rct, it can be noted that the Rp of different samples also exhibits the correlation with VO concentration, which hints that VO is effective in water and hydroxyl adsorption. To figure out this specific effect on water and hydroxyl adsorption induced by introducing VO, DFT analysis and KIE experiments were carried out. As shown in Figure R31, the presence of VO is able to strengthen hydroxyl adsorption and promotes water dissociation which is involved in the rate-limiting process in formate formation. As a result, BOC with VO exhibits higher formate production rate which is beneficial from the reduced energetic span in CO2RR. Overall, the introduction of VO plays a crucial role in modifying the electrochemical interface property through strengthening hydroxyl adsorption accompanied by accelerating charge transfer, which is conductive in promoting water dissociation and boosting CO2RR as a result.

Changes have been made in the revised manuscript (Page 7, Line 170):
" … Next, electrochemical impendence spectroscopy (EIS) was carried out and fitted by equivalent circuit to investigate the electrochemical interface properties 59-60 . With the introduction of VO, the charge transfer can be accelerated, which is beneficial for conductivity and reducing the overpotential (Supplementary Fig. 25 and Supplementary Table 6). Besides, the resistance induced by water and hydroxyl adsorption (Rp) is related with VO concentration, implying that VO might affect the proton transfer by changing water and hydroxyl adsorption. …"     Figure R32, which demonstrates that the range of 200-400 mV dec −1 for Tafel slope is relatively accurate.
Based on our DFT calculations, water dissociation and the first electron transfer are involved in rate-limiting process. If the reaction kinetics is fast, the activity will be improved significantly with lower overpotential and smaller Tafel slope will be observed according to Equation R1. That is, the smaller Tafel slope indicates the favorable reaction kinetics (J. Electrochem. Soc. 169, 106505 (2022); Electrochimica Acta 357, 136840 (2020)).
where, is the overpotential, j is the corresponding current density, a is the Tafel intercept and b is the Tafel slop. The Tafel slope can also be decomposed into where, R is molar gas constant, T is the temperature, α is the transfer coefficient, n is the number of transfer electrons and F is Faraday constant.
Compared with BOC-C, the Tafel slope of BOC-2 is smaller, which implies the faster reaction kinetics. It can be noted that the values of Tafel slope of BOC samples are much larger than 118 mV dec −1 , the typical value which indicates that the first electron transfer is involved in rate-limiting process during 2e CO2RR. As far as we known, there are two major reasons for this large Tafel value as follows: (i) The first reason is that the transfer coefficient for metal oxide offsets the empirical value of 0.5 due to the oxide barrier of metal oxide (Modern aspects of electrochemistry. Springer, Boston, MA, 249-300 (2002); Geochimica ET Cosmochimica Acta 118, 56-71 (2013)). As can be seen in equation R2, it can be concluded that the smaller the transfer coefficient, the larger the value of Tafel slope. Thus, the value of Tafel slope for metal oxide with intrinsic insulating layer will be larger than 118 mV dec −1 , which is consistent with previous works about metal oxide as summarized in Table R10.
(ii) If the chemical step precedes the electron transfer or the chemical step is rate-limiting, the Tafel slope will be much larger. According to our DFT calculations and KIE experiments, water dissociation, as the chemical step without potential dependence, is involved in rate-limiting process and precedes the first electron transfer. To reflect the rate-limiting process more comprehensively, the "virtual energetic span″ (denoted as δEv, ACS Catal. 8, 10590-10598 (2018)), which is the difference between turnover frequency determining intermediate (TDI, CO3* in our work) and turnover frequency determining virtual step (TDTSv, step Ⅳ for BOC), was introduced as the activity determining term. Besides, the Tafel slope is proportional to the derivative of δEv with respect to the overpotential (∂δEv/∂U). As marked in dashed line in Figure   R33, the δEv of BOC decreases with more negative potential applied, indicating the activation energy is affected by applied potential. For BOC with excessive VO (BOC-VO-2), the TDI and TDTSv are CO3* and step Ⅲ , respectively, which manifests that the rate-limiting process is a chemical step without electron transfer. Correspondingly, the value of δEv for BOC-VO-2 in DFT calculations is less affected by applied potentials and the Tafel slope of BOC-4 with more VO in our experiments is much larger than that of other samples.

Changes have been made in the revised manuscript (Page 7, Line 165):
"… Tafel plots were then obtained at sufficient low overpotential to investigate the role of VO in the kinetics of CO2RR. As can be seen from Fig. 3a, BOC-2 shows smaller Tafel slope among all catalysts, indicating that the introduction of VO favors the kinetics of CO2RR 57 . It's noteworthy that the value of Tafel slope is much larger than the reported typical value, which indicates that the chemical step precedes the electron transfer or the chemical step is rate-limiting. The detailed investigation of the reaction mechanism will be discussed later. …" Figure R32. The Tafel slope of BOC-C, BOC-1, BOC-2, BOC-3 and BOC-4. Due to the overpotential of BOC-C is higher than that of other BOC with VO samples, the potential range for BOC-C is a little different compared with other samples (corresponding to Fig.3a in the revised manuscript).

Response:
We thank the reviewer for this suggestive comment. We agree with the reviewer that the most significant change between D2O and H2O is the tunnelling probability. In fact, the theoretical cornerstone of KIE experiments which is considered to reflect water dissociation ability is the H vs. D tunnelling probability. As schemed in Figure R34, the reaction will happen along the potential energy surface in general (route 1). With the consideration of tunneling probability, the particle will move as marked in red arrow (route 2) due to the probability of matter wave (assuming the wavelength λ). The tunnelling probability is positive correlation with λ while λH with small nuclear mass is larger than λD according to de Broglie formula. As a result, the dissociation of H2O requires smaller energy than that of D2O and the reaction rate constant in H2O is theoretically much higher than that in D2O (J. Phys. Chem. Lett. 11, 3724−3730 (2020)). That is, the KIE (the reaction rate ratio between H2O and D2O) will be >1 if water dissociation is involved in rate-limiting process.
As for the effect of the difference between pH and pD value, Lan et al. pointed out that the difference in water dissociation constant (Kw) will influence the dissociation probability at equilibrium (Feq) as shown in equation R3 (J. Phys. Chem. Lett. 11, 3724−3730 (2020)).
where To investigate the effect of viscosity in mass transfer, the finite element method based simulation about the diffusions of CO2 (aq) in H2O and D2O was carried out additionally ( Figure R35). The diffusion coefficients are 1.9×10 −9 m 2 s −1 in H2O and 1.65×10 −9 m 2 s −1 in D2O, respectively. The diffusion thickness of BOC in H2O is slightly larger than that in D2O (5.00 μm of H2O vs. 4.74 μm of D2O). As a result, the current difference is estimated to be 5%-10% when switching the electrolyte from H2O to D2O. Besides, the bulk concentration of CO2 in D2O is a little higher than that in H2O due to the higher solubility (Biochemistry 16, 26 (1977)), which will conversely shrink this diffusion current gap between H2O and D2O. While, the smallest KIE value in our work is of 1.19 which is larger than the diffusion current difference induced by mass transfer. In addition, the KIE experiments were carried out with stirred electrolyte in order to mitigate the limit of mass transport. Overall, the reaction rate ratio (kH2O/kD2O) is predominately affected by water dissociation due to tunneling effect while the effect of pH and viscosity on the KIE value can be negligible.

Changes have been made in the revised manuscript (Page 8, Line 181):
"… The KIE value is calculated by the ratio of formate production rate in KHCO3-H2O and KHCO3-D2O. In general, KIE value is >1 if water dissociation is involved in rate-limiting process of formate production due to the proton tunneling effect while the higher KIE value indicates the greater impact of water dissociation in CO2RR ( Supplementary Fig. 28 and detailed discussion can be seen in Supplementary Information) 60 . …"

Changes have been made in the revised Supplementary Information (Page 2, Line 2):
"…

KIE experiments
Herein, the H2O and D2O were used in electrolytes to investigate the role of VO in water dissociation kinetics. Due to the difference in tunneling probability between H and D, the water dissociation barrier can be different, which affects the reaction rate if water dissociation is involved in the rate-limiting step. To be specific, the reaction of water dissociation will proceed along the potential energy surface in general ( Supplementary Fig. 28a, route 1). With the consideration of tunneling probability, the particle will move as marked in red arrow (route 2) due to the probability of matter wave (assuming the wavelength λ). The tunnelling probability is positive correlation with λ while λH with small nuclear mass is larger than λD according to de Broglie formula. As a result, the dissociation of H2O requires smaller energy than that of D2O 1-2 . If water is involved in RLS, the KIE (the reaction rate ratio between H2O and D2O) will be >1 and the higher KIE value indicates that water dissociation plays a more important role in rate-limiting step. Apart from the H vs. D tunneling probability, the water dissociation is also influenced by water adsorption and OH adsorption. Herein, the water dissociation can be accelerated by improving OH* adsorption and reducing the energetic barrier of water dissociation due to the introduction of VO. Thus, the difference between the reaction rate in H2O and D2O is smaller exhibited as the decreased KIE value.
To investigate the effect of the difference of viscosity between H2O and D2O, we have executed a finite element based simulation about the diffusions of CO2 (aq) in H2O and D2O on planar electrode surfaces [3][4][5] . For H2O, the diffusion coefficient is set to be 1.9×10 −9 m 2 s −1 , and the velocity field in convection is set to be −5×10 −4 m s −1 . As for the case of D2O, we import the Wilke Chang equation to discuss the deviation in diffusion coefficient with the value of 1.65×10 −9 m 2 s −1 in terms of the viscosity coefficient 6 . The results of the concentration distribution are shown in Supplementary Fig. 28b, which indicates that the diffusion thickness of BOC in H2O is slightly larger than that in D2O (5.00 μm of H2O vs. 4.74 μm of D2O). As a result, the current difference is estimated to be 5%-10% when switching the electrolyte from H2O to D2O. Besides, the bulk concentration of CO2 in D2O is a little higher than that in H2O due to the higher solubility, which will conversely shrink this diffusion current gap between H2O and D2O. While, the smallest KIE value in our work is of 1.19 which is larger than the diffusion current difference induced by mass transfer. In addition, the KIE experiments were carried out with stirred electrolyte in order to mitigate the limit of mass transport. Overall, the reaction rate ratio (kH2O/kD2O) is predominately affected by water dissociation probability due to tunneling effect and the KIE value can be used to reflect the water dissociation probability.  .

Comment 4: In-situ Raman studies
The authors conducted in situ Raman studies to investigate the fate of the material during catalysis. The overall Raman experiment is well designed. Unfortunately, the Raman spectra are a bit noisy and overall low in intensity, making it hard to unambiguously assign bands and changes. The discussion of the data is vague and important statements are missing for the reader to understand and follow the rationale. In total, this reviewer believes that the presented in situ Raman spectra and particularly the discussion of the carbonate bands do not conclusively show CO3 adsorption.

Response:
We appreciate the reviewer's valuable comments. The signal to noise ratio in Raman spectra is correlated with acquisition time and accumulation times. To collect the signal in time and reflect the variation more accurately, the Raman spectra were obtained with shorter acquisition time (10 s) and fewer accumulation times (10 times) which can track the whole process during CO2RR with recognizable signals, even though the signal to noise ratio has to be partially sacrificed. Apart from this, reactor design including the electrolyte and optical window thickness affects the signal intensity as well.
Besides, we are sorry that the mechanism discussed here is not clear as we expected. In the revision version, more comprehensive investigation and explanation of mechanism were carried out as can be seen from the responses to the following detailed comments.  Figure R37. The reaction pathway of CO2RR to formate for BOC based on DFT calculation.

The authors concluded the stability of the BOC materials from the largely unaltered Raman band
structure upon going from OCV to cathodic potentials. This seems to be correct. However, the signal intensity is decreased at cathodic potentials. Therefore, the authors should present the Raman spectra at OCV after the potential series to demonstrate that the original spectrum can be re-obtained.

Response:
We thank the reviewer for this valuable suggestion. After a series of potentials applied, the Raman spectra of BOC-2 were collected at OCV again and all these peaks could be kept well without the appearance of metallic Bi signals, demonstrating the stability of BOC-2 under a series of negative potentials ( Figure R38). In addition, it can be noted that the intensity of corresponding signal decreased slightly, which can be attributed to the poor laser tolerance of BOC. In general, the in-situ Raman experiments were carried out by selecting one certain point to monitor the real change during CO2RR.
Thus, the structure of BOC may be destroyed by strong laser after collecting several spectra.
Apart from Raman measurement, the BOC-2 was tested at high current density of 200 mA cm −2 to investigate the stability. After 15 h, the selectivity of formate can be kept without obvious change in bulk phase ( Figure R39), which demonstrates its good stability in CO2RR.

Changes have been made in the revised manuscript (Page 12, Line 272):
"… Then, the Raman spectra of BOC-2 were collected at different potentials (from −0.58 V to −1.08 V) and all these peaks could be kept well without the appearance of metallic Bi signals, demonstrating the stability of BOC-2 under a series of negative potentials (Fig. 5b).

4.3
The interpretation of the 12 CO2 and 13 CO2 experimental Raman spectra is not fully clear to this reviewer. On page 11 line 250, the authors state that adding 12 CO2 results in an 12 CO3 peak. In Figure   5c, the spectral region is marked in blue. The black spectrum shows the intrinsic BOC exhibiting the band at 1067 cm -1 originating from CO3 groups of the BOC. But when 12 CO2 is added only one band is visible (OCV pale blue spectrum) which is low frequency shifted. If the lower frequency band is the 12 CO3 species, where is the band for the intrinsic CO3 of the BOC? Also, it should be clarified how much time was waited between the spectra in 12 CO2 and 13 CO2. The authors should also clarify that the exchange does not occur at potentials lower than the onset potentials.

Response:
We thank the reviewer for this insightful question. The mechanism of CO3 involvement in the formation of formate has been proposed previously but the convincing experimental capture of CO3 species is still in absence (ACS Omega 2, 2561−2567 (2017); ACS Catal. 11, 4988−5003 (2021)).
Inspired by these excellent works, we try to verify the CO3 involvement through theoretical calculations and experimental design.
From the perspective of theoretical investigation on this mechanism, DFT calculations of the whole CO2RR process were carried out at first. During electroreduction, the intrinsic CO3 2− in BOC interacted with electrolyte will spontaneously undergo charge rearrangement and be in-situ transformed into CO3* species with an unpaired electron ( Figure R40). The CO3* is more active and participates in CO2RR to produce formate as the surface species. After that, one oxygen site was left and the input CO2 can be easily adsorbed on oxygen site to form CO3* (due to the downhill process, Figure R41) which proceeds the next cycle ( Figure R42). That is, DFT calculations indicate that CO3* is the surface species participating in CO2RR.
To verify this mechanism in experiments, EPR measurements were carried out benefitting from the presence of unpaired electron in CO3* which can be captured by trapping agent. As shown in Figure   R43, the typical sextet appears during CO2 electroreduction, which implies the presence of CO3*. It can be noted that the CO3* is in absence without CO2, which can be ascribed to that minor CO3* existed in electrode will be consumed rapidly and hard to be captured by trapping agent. During CO2 electroreduction, the CO3* is more available due to the continues input of CO2 and the CO3* can be more easily to be captured as a result.
Then, in situ Raman measurements were further implemented to probe the related species directly along the reaction route. Prior to characterization, the Raman vibrational features of CO3* was calculated and added in revised version as shown in Figure R44.  Figure R45). Apart from that, the signal at 1067 cm −1 is almost disappear accompanied with the appearance of 13 CO3* during 13 CO2 electroreduction ( Figure R46). So, it is reasonable to conclude that the signal at 1067 cm −1 is at least mainly induced by CO3* based on above analysis even though the contribution of intrinsic CO3 2− cannot be totally ruled out.
Finally, in order to verify the cycle of CO3*, isotopic labeling experiments was designed. Ar was purged into the original electrolyte for 15 min to exclude the presence of 12 CO2 ( Figure R46). Next, 13 CO2 was purged into the electrolyte for additional 15 min in order to achieve saturation. At −0.18 V which is far away from the onset potential of formate, only the signal of 12 CO3* can be observed. The absence of OCHO* proves that there is no apparent formate formation at this potential. When a more negative potential of −0.88 V was applied, the signals of 13 CO3* and 12 CO3* were observed simultaneously accompanied by the appearance of O 13 CHO* and O 12 CHO*. After 5 min electroreduction, there were just the signals of 13 CO3* and O 13 CHO*. The overall process implies that CO3* indeed participates in formate formation as the surface species.
With the combination of theoretical calculations and experiments including EPR measurements and in-situ Raman characterization combined with isotopic labeling, it can be clarified that CO3 2− in BOC can be in-situ activated as CO3* during CO2RR which participates in the formate production as surface species. To verify the cycle of CO3* in CO2RR, three steps with isotopic labelling were designed by using 12 CO2 and 13 CO2 as the carbon source interchangeably. First, 12 CO2 was introduced into the reactor, and there is an obvious peak of 12 CO3* accompanied with Bi=O and 12 OCHO* vibration at −0.88 V ( Fig.   5c and Supplementary Fig. 39). Subsequent to Ar purging to remove the residual dissolved 12 CO2 in reactor, 13 CO2 was introduced into the system for 15 min to achieve saturation. It can be noted that a new and wide peak at 1012 cm −1 appears which indicates the presence of 13 CO3* based on the isotopic effect 74 . The slight difference of peak position between experiments and theoretical calculations may be derived from solvent effect and applied potential 75 . Moreover, the intensity of 13 CO3* increases accompanied with the intensity decrease of 12 CO3* as time goes on, indicating that the initial 12 CO3* in the first step is gradually consumed and replaced by 13 CO3*. It's noteworthy that there was no similar exchange at OCV and −0.18 V far away from the onset potential of formate production, which suggests that the observed exchange is indeed resulted from the involvement of CO3* in CO2RR. In the final step, 12 CO2 was fed into the system again and the intensity of 12 CO3* increases impressively with the gradual disappear of 13 CO3*, which manifests clearly that CO3* is involved in CO2RR as the key surface species. …"

Response:
We thank the reviewer for this insightful question and valuable suggestions. The spectra of electrolyte were collected again during the in-situ Raman experiments, and there was no signal can be discernible, which demonstrates the signal of CO3* is exactly from electrode rather than electrolyte ( Figure R47). The absence of electrolyte signal consists with previous works (Nat. Commun. 13, 2039 (2022); Nat. Commun. 11, 3415 (2020)), even though the HCO3 − can be detected by some works. As far as we considered, the signals of HCO3 − and CO3 2− may be too weak to be observed due to the limitation of in-situ Raman cell where the thickness of electrolyte and optical window can influence the intensity of signals to a large extent.
In light of the reviewer's suggestion, the in-situ Raman experiments were carried out specifically in CO2-saturated K2HPO4 to further exclude the influence of HCO3 − . As shown in Figure R48, the CO3* can be well kept accompanied by the signal of OCHO*, which indicates the presence of CO3* rather than the interference of electrolyte. After replacing CO2 by Ar, the signal of CO3* disappears, which indicates that CO3* participates in CO2RR as the surface species. For the ascription of Raman peak of 1067 cm -1 , as discussed in detail in the response of 4.3, this signal is mainly from CO3* which is in-situ transformed from intrinsic CO3 2− in BOC based on our DFT calculations, EPR and Raman characterizations.

Changes have been made in the revised manuscript (Page 13, Line 297):
"… Besides, the signal of CO3* disappears when CO2 was replaced by Ar, which suggests the involvement of CO3* as well. (Supplementary Fig. 40). …" respectively. The green areas represent the characteristic signals of CO3* and the red area is the signal of the OCHO* originated from CO2RR. After replacing CO2 by Ar, the intensity of CO3* decreases notably with the disappear of OCHO* signal, indicating that the involvement of CO3* in formate production (corresponding to Supplementary Fig.40 in the revised Supplementary Information).

4.5
This reviewer appreciates the fact that the comparison to BOC-3 and BOC-C has been made.
However, the interpretation of Fig 5e is  Response: We thank the reviewer for this comment. As discussed above, the charge of CO3 2− can be reorganized and CO3 2− is in-situ transformed into CO3* which participates in CO2RR as surface species.
The partial exchange in BOC-C indicates that there are some CO3 2− fail to be transformed into the active CO3*, which may be one of the reasons for the poor activity of BOC-C. In addition, it can be noted that the intensity of residual signal at 1067 cm −1 during 13 CO2RR (Line 4) is much lower than that during 12 CO2RR (Line 2), which implies that CO3* is dominate in the contribution of 1067 cm −1 signal ( Figure   R49).

Changes have been made in the revised manuscript (Page 13, Line 295):
"… The partial exchange in BOC-C after 13 CO2RR indicates that there are some CO3 2− fail to be transformed into the active CO3*, which may be one of the reasons for the poor activity of BOC-C. …" Figure R49. In situ Raman spectroscopy with 12 CO2 and 13 CO2 as carbon source for BOC-C. 1) In Figure S1, there are several XRD peaks. BOC nanosheets appear to be polycrystalline rather than single crystalline.

Response:
We thank the reviewer for this comment. As can been seen from Figure R50a

Changes have been made in the revised manuscript (Page 3, Line 88):
"… A lattice distance of 0.275 nm corresponding to (110) plane is clearly discerned in highresolution TEM (HRTEM) images (Fig. 1c), while the selected area electron diffraction (SAED) patterns display the BOC nanosheets are consisted of individual single crystalline nanosheet (inset in Fig. 1c and Supplementary Fig. 3d). …"  2) The reviewer cannot find the defect in Figures 1d and S5. Please explain how to identify one.

Response:
We thank the reviewer for this comment. As shown in dark field TEM image ( Figure R52 Figure 1g, the y-axis values show less than 3% difference in intensity that might be within the experimental error. Also, why the presence of oxygen vacancy shows a larger intensity in the y-axis than that of oxygen sites?

3) In
Response: Thanks for this valuable comment. With the model of bright-filed STEM, the atoms are dark with the bright background. If there is no atom, the intensity would be higher due to the bright background. That's why the intensity of oxygen vacancy is relative higher than that of oxygen site. As shown in Figure R53, the relatively large black hole is the bismuth atom while the small hole can be assigned to oxygen atom due to the difference in atomic number. To eliminate the effect of error, three areas were selected as shown in Figure R54 and the intensity of oxygen atom is similar, with a maximum difference of 2000 a.u. (section 2). In contrast, the difference between oxygen vacancy and oxygen atom is more than 6000 a.u., much larger than the error of oxygen atom intensity (section 1 and section 3).

5) If the water dissociation is promoted, hydrogen evolution reaction (HER) might also be enhanced
due to the increase in the number of protons near the surface. So, please discuss why the competing HER is not enhanced but only CO2 reduction is enhanced.

Response:
We thank the reviewer for this insightful comment. We agree with the reviewer's opinion that the promoted water dissociation is also beneficial for HER. The jhydrogen of BOC with VO is slightly larger than that of BOC-C ( Figure R56). However, the BOC-4 with more VO and sluggish water dissociation kinetics shows larger jhydrogen compared with BOC-2 with accelerated water kinetics, which indicates that the production rate of hydrogen is not merely restricted by water dissociation. In fact, the kinetics of HER is predominately restricted by H* adsorption as demonstrated by previous works (J. In order to get further insight into the H* adsorption, DFT calculation was added. As can be seen in Figure R57, the energy of H* adsorption on BOC is much larger than CO2 adsorption, suggesting the weak H* adsorption of BOC and CO2RR will occur preferentially compared with HER. Therefore, the HER performance of BOC with VO is relatively poor compared with that of CO2RR, even though beneficial from the accelerated water dissociation. On the other hand, the formate production rate increases sharply by promoting water dissociation because the formate production is mainly limited by water dissociation as discussed in our work. Thus, it's an effective method to promote CO2RR through promoting water dissociation although concomitant HER performance is improved slightly. Changes have been made in the revised manuscript (Page 11, Line 231): "…As can be deduced from Fig. 4e, the energy variation of the step Ⅲ is mainly from ΔμOH−w, which suggests that the formate production rate is dominantly restricted by the sluggish water dissociation. While the introduction of VO accelerates the kinetics of water dissociation by notably reducing ΔμOH−w, which largely decreases the energy barrier in the formation of OCHO*. These results are in good agreement with KIE experiments discussed above. On the other hand, the accelerated water dissociation kinetics has minor effects on HER due to the poor H* adsorption on BOC ( Supplementary   Fig. 34).…"

6) Authors claim that "there is no obvious change in phase structure, morphology and valance state,
which demonstrates the stability of BOC-2 ( Supplementary Fig. 15 Figure R58). Second, the in-situ Raman measurements were conducted for BOC-2 at a series of negative potential to monitor the stability of BOC-2 on surface. As can been seen from Figure Figure R60). That is, the Bi 3+ tends to be reduced to metallic Bi in thermodynamics at negative potentials while Bi 0 is easily to be oxidized to be BOC during CO2RR due to high local pH and accessible CO2 adsorption.

Changes have been made in the revised manuscript (Page 7. Line 146):
"… To relieve the limitation on mass transfer and pursue commercial current density requirements, BOC-2 was further integrated into gas diffusion electrode (GDE) and evaluated in flow-cell system 53 .
The jformate of BOC-2 is 1.6 times as high as that of BOC-C at −1.68 V while the turnover frequency can be up to 0.72 s −1 at 200 mA cm −2 , which demonstrates the activity enhancement through introducing VO can also be achieved in flow-cell system (Supplementary Fig. 17 and Supplementary Table 4). In addition, a stable FEformate of 80% at large current density of 200 mA cm −2 can be observed during 15 h, which indicates the potential for practical application (Supplementary Fig. 18). Even though Bi 3+ could be reduced at negative potentials in thermodynamics, the good stability of BOC can be explained by the spontaneous CO2 adsorption and high local pH where the oxide state of Bi is stable 29, 33, 54-55 .
Moreover, 13 CO2 labeling experiment was carried out and the proton doublet resulting from H-13 C coupling and H 13 COO − is observed in 1 H NMR and 13 C NMR, respectively ( Supplementary Fig. 19), demonstrating that the produced formate derives from CO2 56 . …" Figure R58. a The stability test of BOC-2 at 200 mA cm −2 . b XRD patterns of BOC-2 before and after stability test, where there is no characteristic peak of metallic bismuth phase (corresponding to Supplementary Fig.18 in the revised Supplementary Information).  7) The reviewer also wants to know and recommend explaining why H2O showed a higher formate production rate compared to D2O in Figures 3b and 3c.

Response:
We thank the reviewer for this valuable comment. Due to the difference in tunneling this work. In light of the reviewer's suggestion, TEM and corresponding SAED measurements on BOC-2 after stability test were implemented to monitor the crystal structure state. As shown in Figure R1, BOC-2 exhibits clear lattice fringes and single crystalline property while lattice disorder induced by VO can be observed as well, which is quite similar to BOC-2 before CO2RR ( Figure R2). This indiscernible variation in SEM and TEM indicates that there is no obvious change in morphology and crystal structure in BOC-2 after stability test.
Moreover, the surface state of BOC-2 after evaluation was also studied. As can be seen from Figure R3a, the chemical state of Bi for BOC-2 after CO2RR can keeps well, indicating the stability on the surface of BOC-2 with VO. This result is consistent with our in-situ Raman experiment ( Figure R3b). Overall, the BOC-2 shows good stability during CO2RR without obvious change in morphology, crystal structure and chemical state. These supplementary characterization results have been added in the revised Supplementary Information.
On the other hand, even though the repeated redox reaction of Bi may happen during CO2RR, the undetectable change in XRD, SEM, TEM, XPS and Raman characterizations suggests that this redox is a fast and reversible process, which manifests the good stability of BOC-2. While the possible variation is hard to be observed from ex-situ characterizations. Therefore, more insitu time-resolved characterizations need to be developed in this field, and we will make more efforts to study the mechanism of stability in the future.   .